diff --git a/src/data/CCRAL.ml b/src/data/CCRAL.ml
index a8e7422d..351e687b 100644
--- a/src/data/CCRAL.ml
+++ b/src/data/CCRAL.ml
@@ -342,6 +342,10 @@ and drop_tree_ ~size n t tail = match t with
       then drop_tree_ ~size:size' (n-1) l (append_tree_ r tail)
       else drop_tree_ ~size:size' (n-1-size') r tail
 
+(*$T
+  of_list [1;2;3] |> drop 2 |> length = 1
+*)
+
 let drop_while ~f l =
   let rec aux p st = match st with
     | St_nil -> Nil